1. Field of the Invention
The present invention relates generally to Sagnac interferometers and more particularly to detecting the eigen frequency of a sensing coil of a fiber optic gyroscope.
2. Background of the Invention
The eigen frequency (sometimes called the “proper” frequency) of a fiber optic gyroscope (FOG) sensing coil is an extremely important parameter for the operation of navigation and high performance grade FOGs. The eigen frequency is essentially defined by the optical path length of the sensing coil. Many sources of rate output errors are reduced or effectively eliminated by operating the bias modulation at the eigen frequency. Generally speaking, the bias modulation must be operated to within at least a few ppm of the eigen frequency in order for FOGs to meet high performance requirements. New applications are now putting more demanding performance requirements on these types of gyros.
For navigation grade FOGs that must operate over wide temperature ranges, the current state of the art method of maintaining the bias modulation at the eigen frequency employs a method of indirectly measuring the eigen frequency. The eigen frequency is estimated from temperature measurements of the coil and calibration coefficients on a calibrated lookup table. This method has drawbacks in that it requires, among other things, extensive testing for calibration, and, since it is an indirect measurement of the eigen frequency, it has accuracy limitations.
High performance FOGs for space-based pointing and submarine navigation applications currently use temperature control of the fiber coil to maintain the eigen frequency near a constant value. The bias modulation frequency is then maintained near the eigen frequency by deriving the frequency from a temperature controlled or compensated crystal oscillator. This method works well for maintaining gyro operation at the eigen frequency for a relatively short duration of one to two years and under a relatively benign laboratory environment. However, the drift of the crystal oscillator over many years (e.g., 10 or greater) of operation is typically greater than what is required for high performance. Furthermore, it is possible that the coil eigen frequency will significantly drift over many years due to aging effects of the fiber coil itself.
For both navigation and high performance grade FOGs, a better method for controlling the bias modulation to the eigen frequency is to employ an eigen frequency detector and a servo control loop that automatically maintains the gyro operation at the eigen frequency. The eigen frequency detector provides an error signal that is null when the bias modulation is at the coil eigen frequency. The servo loop maintains the bias modulation at the coil eigen frequency by nulling the error signal from the eigen frequency detector.
Eigen frequency detection can be accomplished by a number of methods. One such method, described in U.S. Pat. No. 5,090,809, involves demodulating the quadrature of the rate signal to extract information about the eigen frequency. This method relies on imperfections of the phase modulator in the form of spurious intensity modulation to generate an eigen frequency error signal. A drawback to this approach is that the sensitivity of the eigen frequency error signal to changes in eigen frequency depends on imperfections of an optical component, which may or may not be at an acceptable level and may vary drastically from unit to unit. To overcome this drawback, this patent teaches that it may be desirable to increase the magnitude of the spurious intensity modulation by decreasing the length of the phase modulator or by adding an intensity modulator to the optical circuit. This method adds significant complexity to the optical circuit and introduces additional intensity modulation, which could generate other types of gyro errors. Another drawback to this approach is the detection process which involves demodulating the quadrature of the rate signal to extract the eigen frequency error signal. Since the demodulation process of a practical device cannot be made to be exactly in quadrature (90 degrees out of phase) of the rate signal, some crosstalk will occur between the rate and the eigen frequency error signal channels. This crosstalk will limit the performance of the eigen frequency servo. Furthermore, since the eigen frequency demodulator is operating at the same frequency as the rate demodulator, then the eigen frequency demodulator will be sensitive to any interference at the same frequency. This interference can be in the form of electrostatic or electromagnetic coupling, or signal currents causing spurious signals due to voltage drops on ground lines or power-supply lines. Signal interference can also limit the performance of this type of eigen frequency servo.
Another approach is describe in U.S. Pat. No. 5,734,469, which is an improvement over U.S. Pat. No. 5,090,809. This approach also involves demodulating the quadrature of the rate signal. However, the sensitivity of the eigen frequency error signal to changes in eigen frequency is determined and increased by implementing a square-wave bias modulation with a non 50—50 duty cycle. The advantage of this method is that changes in the optical components are not required to enhance the sensitivity of the eigen frequency error signal. However, this invention has the same disadvantages associated with quadrature detection that were mentioned in the previous paragraph for U.S. Pat. No. 5,090,809. Furthermore, laboratory tests performed have shown that this method, and thus any method involving quadrature detection, may not be accurate enough for the high performance applications.
Accordingly there remains a need to provide a scheme by which the eigen frequency of a fiber loop of a FOG can be easily and accurately detected so that FOGs with the desired stability and performance can operate over wide temperature ranges for long periods of time.